1. Field of the Invention
The present invention relates to a silicon cleaning method for semiconductor materials, to a polycrystalline silicon chunk and to a silicon cleaning device.
2. Description of the Related Art
The single crystal silicon wafers used for semiconductors are produced by melting high-purity polycrystalline silicon (referred to as polysilicon) as a raw material, pulling the single crystal silicon from the molten polysilicon by a method commonly referred to as the CZ method (Czochralski method) and processing this into wafers. The polycrystalline silicon serving as the raw material of the single crystal silicon is mainly obtained in the form of rod-shaped ingots by a CVD reaction in a water-cooled bell jar which is commonly referred to as the Siemens method. These polycrystalline silicon ingots are sent to a single crystal pulling process after going through various processes consisting of cutting, crushing, classifying, acid cleaning, pure water rinsing, drying and packaging, which are conducted in that order.
As has been described above, bulk polycrystalline silicon, which is obtained by cutting, crushing and classifying the polycrystalline silicon ingots, is subjected to acid cleaning and pure water rinsing to make sure of removal of contaminants. The acid cleaning uses a mixed liquid of hydrofluoric acid and nitric acid to completely remove contaminating impurities adhered to the surface of the polycrystalline silicon, and an oxide film on the surface of the polycrystalline silicon chunk is removed by the mixed liquid. The acid cleaning is followed by rinsing with pure water, drying and packaging to obtain shipped products.
The aforementioned pure water cleaning is the final step of the cleaning process, and the pure water used for cleaning is required to be of high purity and, if possible, not any contain impurities. For example, in the pure water cleaning of wafers, a pure water supply system for obtaining high-purity water is known in which the untreated water is passed through activated charcoal to remove high molecular weight organic substances and residual chlorine, this water is then passed through a cation exchanger and anion exchanger to remove other residual ion components such as metal ions, and finally the water is passed through a reverse osmosis apparatus to remove ion components, organic substances and fine particles (see “Silicon Chemistry”, pp. 903–908, 1996, Realize Corp.). In addition, as an example of a typical high-purity water production system, a water purifying apparatus has been proposed that uses a reverse osmosis membrane unit while also providing an ion exchange unit for the final filtration member (see Japanese Unexamined Patent Application, First Publication No. 2003-245667).
In the field of semiconductors, increasingly severe quality requirements are being placed on the raw material single crystal silicon accompanying the increasing levels of integration degree. Particularly, requirements for reducing impurities are becoming extremely severe. For example, a method is required wherein the method can decrease impurity concentrations to levels lower than that by the pure water cleaning method of the prior art using the aforementioned pure water supply system (see the aforementioned “Silicon Chemistry”). On the other hand, the water purifying apparatus of the prior art that combines a reverse osmosis device and ion exchange device (see Japanese Unexamined Patent Application, First Publication No. 2003-245667) is intended to be used for the purpose of obtaining highly pure water efficiently, easily and inexpensively. However, an action of removing specific metal ions is not at all discussed in the prior art of the water purifying apparatus.